(a) Field of the invention
The present invention relates to a scanning type optical microscope which is based on the system for conducting the scanning of an object with a beam of light.
(b) Description of the prior art
There have been proposed scanning type optical microscopes which are of many advantages over ordinary microscopes that images of good contrast are obtained because of the absence of scattered lights coming in various other directions than from the picture elements per se which are the target for observation, or that special and effective images can be formed by relying on such techniques as the confocal method or the differential phase difference method, or further that various kinds of physical phenomena can be imaged such as OBIC (Optical Beam Induced Current) images, photo-acoustic images, etc.
As the scanning system employed in the scanning type optical microscopes, there is the system that the specimen for observation is mechanically moved for scanning, and the system that a laser beam spot per se is moved to scan the stationary specimen. In case of the system arranged so that the observation is performed while mechanically moving the specimen, however, there are the drawbacks that the specimens are limited to only those having a small size and a light weight or those which are fixed to inhibit their movement caused by vibrations developing from the scanning operations, and further that the scanning cycles cannot be set at a much high rate. In view of these drawbacks encountered in the prior-art such microscopes, the inventor has proposed, in his U.S. Pat. No. 4,734,578, a scanning type optical microscope which, while being of the system to move the laser beam spot, is capable of forming such special images as obtained from the confocal technique, the differential phase difference technique, etc. and which allows the observation of specimens of any kind.
Description will be made hereunder with respect to this unique system by referring to FIGS. 1 to 5.
This scanning type optical microscope is so arranged that, by introducing into the optical microscope, the system that the surface of the specimen is scanned by deflecting the light beam with a light-deflector, a good convenience of its handling is secured as in the ordinary microscopes while retaining a high degree of resolving power. Also, by setting the light-deflector at the position of the pupil in the scanning optical system, the optical axis in the scanning system can be held constant even when the scanning by the light beam is performed by the light deflector. And, along therewith, by arranging, in case of detection of the transmitted light, the detector at a position conjugate with the pupil, it is made feasible to utilize the informations occurring at the pupil in case of off-axial light rays also. Whereby, it has been made possible to conduct an observation by a mere manipulation of a switch of the electric circuitry even in case of such a special microscopy as mentioned above.
FIG. 1 is an illustration showing the arrangement of the scanning optical system and the detector of the above-mentioned prior art which takes the pupil into consideration. A light beam 1 coming from a laser source which is considered equivalently as the spot light source transmits through a beam splitter 2 and incides onto a first light-deflector 3. This light-deflector 3 is disposed at a position conjugate with the pupil 5 of an objective lens 4. In case no deflection of light is performed, the light beam 1 advances along an optical axis 6. In case light deflection is performed, i.e. in case scanning is performed by the light beam 1, it should be noted that, since the light-deflector 3 is provided at the position of the pupil, the direction of the light beam 1 is rendered to the coincident with that of an off-axial principal ray 7, and the center of the light beam 1 also is rendered to be coincident with the off-axial principal ray 7. Next, in each of these cases, the light beam passes through pupil relay lenses 8 and 9 and incides onto a second light-deflector 10 which is disposed also at the position of the pupil. When this light-deflector 10 is to perform the scanning only in the direction X among the two-dimensional scanning, the first-mentioned light-deflector 3 will undertake the scanning in the direction Y. By the employment of a light-deflector which is capable of performing the deflection of light in both directions X-Y, therefore, the provision of a single light-deflector is enough. The light beam which thus scans in two dimensions by the two light-deflectors 3 and 10 is caused to incide onto the pupil 5 of the objective lens 4 by means of a pupil projection lens 11 and a focusing lens 12. An off-axial light beam which is formed by the light-deflectors 3 and 10 also has its direction and center coincident with the off-axial principal ray 7. Therefore, this off-axial light-beam also impinges exactly onto the pupil 5 of the objective lens 4. And, these light beams develop, on a specimen 13 by the objective lens 4, a dot-like spot of light which is restricted by diffraction. By performing scanning in both directions X and Y by the light-deflectors 3 and 10, a two-dimensional scanning of the specimen 13 by the dot-like light is carried out.
In case the light which has transmitted through the specimen 13 is observed, the light is collected by a condenser lens 14 and the resulting light is detected by a detector 15. This detector 15 also is disposed at the position of the pupil. Accordingly, off-axial lights appear always at a same position, whereby it is possible to prevent the adversary effects caused by, for example, uneven sensitivity of the detector 15. Also, the area for the installation of the detector 15 can be greatly reduced. Furthermore, in case a differential type detection is performed, the detector 15 is formed with two detector-constituent devices 15a and 15b, and they are disposed symmetrically relative to the optical axis 6. In this case, setting is made to establish the condition that, even in the event of an off-axial light, the center of the beam stays coincident with the off-axial principal ray, whereby the detector-constituent devices 15a and 15b assume symmetrical positions relative to the off-axial principal ray also. Thus, it is possible to perform a precise differential type detection.
Also, in case detection is conducted with the reflection light coming from the specimen 13, the light beam which has been reflected at the specimen 13 transmits through the objective lens 4 and its pupil 5, and further passes through a focusing lens 12, and is focused once. This focal plane is the one which is used in ordinary optical microscopes to observe an image. Furthermore, the light beam is caused to return to the light-deflector 10 by the pupil projection lens 11. In this way, the reflection light beam returns to the beam splitter 2 by tracing back exactly the same course as that taken by the beam of light when it initially incided onto the specimen. Therefrom, the reflection beam is derived by the beam splitter 2 to become a detection beam 16. Since the reflection beam has returned after passing through the light-deflectors 10 and 3, an off-axial scanning will not affect this detection beam 16 in any way. The detection beam 16 is then squeezed into a dot-like form by a light-collecting lens 17. Therefore, by the provision of a pin-hole 18 at the position where the beam is squeezed into a dot-like form, and by performing a detection by means of a detector 19 which is located rearwardly of the pin-hole 18, it is possible to obtain a flare-free image of a higher resolution than offered by an ordinary microscope. It will be needless to say that a normal image can be obtained even where the pin-hole 18 is not provided. Also, by the provision of a black dot-like light-blocking member at the position where the light beam is squeezed into a dot form, it is possible to easily observe a dark-field image. Also, by constructing the detector 19 with two detector-constituent devices 19a and 19b, and by disposing them at positions of the expansion of the light beam in symmetrical fashion relative to the optical axis, it is possible to conduct a differential type observation. Here, it will be needless to say that the signal supplied from the detector 19 can be converted to a visible image by such an indicator as a CRT.
Next, description will be made hereunder in further detail with respect to the need for considering the position of pupil in case of the optical system and the detection system for scanning with a light beam. FIG. 2 shows the instance wherein the light-deflector 3 is not provided at the position 20 of pupil in that region of the light-deflector 3 and of the pupil relay lens 8 of FIG. 1. When the incident beam 1 is deflected by the light-deflector 3, the center 21 of this deflected light beam is not coincident with the off-axial principal ray 7 which is determined by the objective lens 4. This indicates that the off-axial light beam does not precisely incide onto the objective lens 4. In FIG. 3, numeral 22 represents the pupil of the objective lens 4. It is shown that the center of this pupil 22 is either the optical axis 6 or the off-axial principal ray 7. In this case, when the light-deflector 3 is provided at a position conjugate with the pupil, the scanned off-axial light beam coincides with the off-axial principal ray 7, and it precisely incides onto the pupil 22 of the objective lens 4. In contrast thereto, when the light-deflector 3 is not provided at the position of the pupil, the center 21 of the light beam is not coincident with the off-axial principal ray 7, so that the expansion 23 of the light beam becomes as shown in FIG. 3, and this expansion of light beam will be subjected to vignetting without exactly impinging onto the pupil 22. In this case, by arranging the incident beam to be a large light beam like the expansion 23', there will not arise a shortage of the amount of light, but nevertheless this is not appropriate for utilizing the pupil informations.
Next, description will be made of the instance wherein the detector is not provided at the position of the pupil in the detection of the transmitted light. In FIG. 4, the light beam is projected in a dot form onto a specimen 25 by an objective lens 24, and the transmitted light beam is detected by detectors 27 and 28 which are disposed symmetrically relative to an optical axis 26. In case of the system for conducting the scanning by moving the specimen, the light beam is always located on the optical axis. Therefore, it is always possible to perform the differential type detection. However, in case of scanning with a light beam by means of a light deflector, there occur off-axial lights. Therefore, unless the detectors are provided at the position of the pupil, the positions of the detectors 27 and 28 will not become symmetrical relative to the off-axial principal ray 29. As shown practically in FIG. 4, the off-axial principal ray 29 is produced on the detector 28. Accordingly, it is not possible to obtain an accurate differential image. From the foregoing description, in the scanning type optical microscope using the system of conducting the scanning with a light beam, there is the necessity for setting a light-deflector at the position of the pupil of the optical system and for providing a detector also at the position of the pupil. By so arranging, special microscopy can be accomplished easily, and also there can be obtained an image of a high resolution. However, as is apparant from the foregoing description, it should be noted that, in case detection is performed with a reflection light, this reflection light again passes through the light-deflector, so that there is no restriction on the position of the detector.
The above-mentioned example of the prior art is so arranged that, also in the system of scanning with a laser beam spot by the provision of a scanning means (light-deflector) and a detector at the position of the pupil, there can be obtained an accurate differential phase image. In case, however, the microscope to which this sysem is applied is an ordinary optical microscope, it is the usual case that the position of the pupil of the objective lens differs depending on the magnification or the type of the objective lens. Therefore, in case the scanning means and the detector are set in accordance with the position of the pupil of a given objective lens, there would occur the instance, when a different objective lens is used, that the detector becomes displaced from the position of the pupil. Also, there could occur a displacement of the position of the pupil arising from the setting error of the detector or from the restrictions of placement of the detector. For this reason, the amount of light of the light beams incident to the two detectors intended for the detection of differential phases, e.g. the detectors 15a and 15b of FIG. 1, will vary, respectively, depending on the height of the image as shown in FIGS. 5A and 10B.
Let us here consider the case that, for example, the two detectors have their sizes which are substantially larger than the size of the projected pupil. Assuming that the radius of the pupil as p, the amount of displacement of the pupil from the optical axis as .delta., and the amount of light when there is no displacement of pupil as
1. Then, the amount of light f(.delta.) will become: ##EQU1##
Here, when the sum of the output signals of these two detectors is calculated to obtain a normal image, these two signals cancel out each other so that there will develop no change in the amount of light attributable to the image height. In case, however, it is intended to obtain a differential image, the difference between the output signals of the two detectors is calculated. Thus, the change in the amount of light due to the image height will become doubled. In this latter case, there arises the inconvenience that, when, for example, the direction of the boundary line between the two detectors is set normal to the direction of the horizontal scanning, i.e. in case arrangement is given so as to obtain a differential image in the direction of the horizontal scanning, the brightness will differ on the left-hand side of the picture field from the right-hand side thereof.
Such a difference of brightness between the areas on the two sides of the picture field will not cause a substantial problem in case the image height is small or where the contrast of the differential image is not stressed. In case, however, the image height is big or in case the contrast of the differential image is to be stressed, the abovesaid unevenness of the brightness will consitute a substantial problem. For example, there could arise such an inconvenience that the left-hand margin of the picture field is excessively bright so that the details of the differential image are lost, whereas the right-hand margin is too dark and nothing can be observed.